1. Field of the Invention
The present invention relates to a projection optical apparatus which is used, for example, in a semiconductor wafer printing apparatus for semiconductor fabrication to project a desired pattern and a photographic mask or reticle adapted for use therewith.
2. Description of the Prior Art
With projection optical apparatus heretofore used for photolithographic purposes, it has been the usual practice that the light transmitted through a photographic mask or reticle containing a pattern of opaque and transparent areas (hereinafter generally referred to as a mask) is utilized to print the pattern on a semiconductor wafer. Then, masks have been used in which a transparent substrate made of glass or the like is formed with a desired pattern by using a light blocking material such as chromium. When a mask of this type is illuminated from its one surface side by a suitable light source, the light is transmitted to the other surface side through the mask except the pattern portion. The transmitted light is projected to a desired projection optical system and thus the pattern is projected onto a given wafer.
Then, with this type of projection optical apparatus, if any dust, fingerprint or flaw exists on the portion of the mask substrate through which the light is to be transmitted, the light to be transmitted is intercepted. As a result, the pattern of the dust or the like is also projected with the resulting detrimental effect on the essential pattern to be projected. This results in the formation of defective elements or circuit on the semiconductor wafer or the like and the production efficiency is decreased.